For the OS/360 object files, I don't believe there is anything
that the assembler can't produce, but with LE and GOFF, there are
some things. I don't remember them, and a web search doesn't
show them.

Well, the assemblers have PUNCH which lets you write any bits
you want into the object file. That is sometimes used to generate
linker control cards into the object deck, and also to generate
the SYSGEN files, which all come from the assembler.